This invention relates to radioactivity well logging and more particularly to an improved method and system for carrying out epithermal neutron lifetime measurements in a time-dependent radioactivity borehole logging tool.
In neutron-neutron logging, a source of primary radiation irradiates the formations surrounding a borehole with neutrons. The resulting secondary radiation is measured by detectors axially spaced from the source within the borehole. Such secondary radiation includes epithermal neutrons, thermal neutrons, and thermal neutron capture gamma rays. U.S. Pat. No. 4,005,290 to Allen discloses a borehole logging tool, a neutron source, and epithermal and thermal neutron detectors. Both the epithermal and thermal neutron detectors are of the proportional counter type disclosed in U.S. Pat. No. 3,102,198 to Bonner and filled with helium-3 gas at superatmospheric pressure preferably in the order of about two to twenty atmospheres absolute. The epithermal neutron detectors are further surrounded with cadmium shielding to prevent thermal neutrons from reaching the active volume of the counters. At least two electrodes in electrically conductive contact with the helium-3 gas are connected with a voltage source to establish an electrostatic field for the collection of ionized gas particles produced in the zone of the helium-3 gas upon entry of epithermal neutrons from the surrounding formations. U.S. Pat. No. 4,241,253 further discloses a neutron moderating material located between the helium-3 filled neutron counter and a thermal neutron shield. The moderator slows down epithermal neutrons penetrating the thermal neutron shield to enable their counting by the neutron counter.